Torque tube sizing for stiffness

The drawing shows an adapter to secure a container to arms which connect to a moving dolly (not shown). A 6x6 beam has two arms welded to the ends that support the container at the sides and two opposing arms (4" deep and 8" wide) offset 2' toward the center that connect to the dolly. The dolly axle (8000 lb max) is 4 feet from the end of the container. The container side arms are pinned at the bottom corner sockets. Chains hooked to the container top corners connect via chain binders (not shown) to a 7/16" cable routed through guides on the arms and under the container. The cable should not contact the bottom of the container. It is strictly anti torque. This arrangement keeps the arms secure to the sides of the container. The concept is to let the cables oppose the twist but not support the bottom of the container which can not take a load. This arrangement twists the beam in the two foot section between the ends and the dolly connection arms. The support assumes positive g's. The moving dolly axles can support 8000 lbs at 4 feet from the container which I translate to 16000 ft lbs of torque on each arm.

It is important to keep adapter weight low.

Questions:

1. How can I determine if a 6x6x1/4 inch hollow mild steel tube that is 8 feet long is stiff enough to keep beam twist low (maybe one degree which I estimate would move the axle about an inch) at 16000 foot lbs of torque?

2. How can I determine how much the square tube could rotate (radians) per foot length to provide a safety factor of 5 at 16000 foot lbs of torque?

3. How do I determine the length of the outer arms to minimize the required strength of the anti torque cables?